An optical receptacle is used as a component for optically connecting an optical fiber connector to an optical element such as a light receiving element, a light emitting element, etc., in an optical module of an optical communication transceiver.
In recent years, it is necessary to increase the speed of optical communication transceivers as IP traffic increases. Generally, the configurations of the transceiver and the like that employ the receptacle-type optical module are standardized; and it is necessary to downsize the optical module because the space necessary for the electronic circuit increases as the modulation rate of the optical signal emitted from a semiconductor laser which is one optical element is increased.
The mode field diameter of the semiconductor laser element is smaller than a core diameter of 10 μm of the optical fiber generally used as the transmission line of the optical signal.
In recent years, optical modules also are being used to increase the communication speed of optical transceivers, where the optical module has a structure in which multiple semiconductor lasers are included inside a single module; the light that is emitted from each of the semiconductor lasers is multiplexed in one waveguide inside the optical waveguide formed in the interior of a plate-like member; and subsequently, optical coupling is made to the optical fiber of the optical receptacle. To downsize these optical modules, it is necessary to downsize the plate-like member having the optical waveguide described above; and there is a trend of the core diameter of the optical waveguide becoming smaller.
In an optical module in which a light receiving element is used instead of the light emitting element as well, there is a trend of reducing the light-receiving diameter of the light receiving element for use in higher-speed longer-distance communication applications.
In the case where there is a difference between the fiber core diameter and the mode field diameter of the optical element, while it is necessary for the lens to have a magnification function to condense the light emitted from the semiconductor laser elements into the fiber core or condense the light emitted from the fiber core into the light receiving element, there is a problem that the larger the difference, the longer the focal length of the lens or the larger the number of necessary lenses, which makes the optical system complicated and expensive.
A method is known that in order to increase the total length of the module or to prevent complication of the optical system, the magnification by the lens is kept small, and instead a lens is formed at the tip of a part of the end face of the optical fiber on the optical element side, or the GI fiber is fused, thereby enlarging the mode field diameter of the incident light and making the optimum mode field diameter for the fiber incident on the fiber end face (for example, JP 2006-154243 A (Kokai)).
However, since the method of JP 2006-154243 A (Kokai) uses a GI fiber whose mode field diameter periodically changes, in order to obtain an optimum mode field diameter, it is necessary to strictly control the length of the GI fiber, and thus there is a problem that it is difficult to manage manufacturing.
When fibers having gradually different refractive indexes are fusion spliced from the core center to the outer circumference like the GI fiber, in the fusion splicing technique in which the fiber end faces are melted and integrated, the cores with different refractive indices are melted and blended. Therefore, it is difficult to manage the refractive index around the fusion bonded portion and there is a problem that the optical loss increases.